# Probability distributions from a linear regression model

I've constructed a Linear Regression model (OLS) and I'm wondering what the statistical certainty is for the response variable to be true.

Say I'm collecting data from a biological system and I want to predict the weight of the cells, e.g resulting in y=50mg. I want to figure out how certain I am that y=50mg is true.

Also, if I assume intervall values 1-10mg, 11-20, 21-30.. 91-100mg - can I assume linear distribution i.e that if y=50mg then it's a 10% chance that the values is between 41-50mg (and nothing more) or am I equipped to assume trapezoidal probability function i.e that it's a 50% chance that the value is 41-50mg or less.

Any thoughts on this?